[Lazarus] how to set two memos for parallel text editing ?

[Etienne Leblois]

Dear list,

I want to edit two textfiles in a parallel way, so that visible portion 
of text one in memo1 has always same lines extent as the visible portion 
of text two in memo2, say lines n1 to n2.

Of course WordWrap must be false, so that one_text_line=one_visible_line

Then, how to manage that scrolling or arrowing up in one memo will also 
scroll up the other one ? Intercept event yes, but how to issue/code a 
scrolling order for memo that does not have focus ?

Thanks for hints,

Etienne.